1. Technical Field
The present invention is directed to a process and apparatus for depositing thin films made of a composite material formed from at least two gaseous reactants, wherein the interaction of the reactants must be controlled in order to obtain a film of the desired quality.
2. Art Background
Composite films, i.e. films of materials formed from at least two different elements such as silicon nitride (Si.sub.3 N.sub.4), silicon dioxide (SiO.sub.2), aluminum oxide (Al.sub.2 O.sub.3), aluminum nitride (AlN), and titanium oxide (TiO.sub.2) are formed by introducing reactive gases into a chamber. The reactive gases are precursor gases for films of the desired material. Since the gases react to form the desired composite material, the reactions must be controlled in order to obtain a film of the desired material.
Composite films are formed using techniques such as Chemical Vapor Deposition (CVD). In CVD, a non-volatile solid film is formed on a substrate by a surface-pyrolized reaction of the gaseous reagents. A typical CVD reaction process comprises the following steps, (1) gaseous reagent and inert carrier gas are introduced into the reaction chamber, (2) gaseous reagent is transported by convection and diffusion to the surface of the substrate, (3) reagent species are absorbed onto the substrate where they undergo migration and film forming reactions and (4) gaseous byproducts of the reaction and unused reagents are removed from the chamber. The pressure in the deposition chamber may be atmospheric or reduced as low as a fraction of 1 Torr, as in the respective cases of Atmospheric Pressure CVD (APCVD) or Low Pressure CVD (LPCVD). The energy required to drive the reaction is supplied as heat to the substrate. For practical reaction rates, substrates are typically heated to temperatures ranging from about 500.degree. C. to as high as 1600.degree. C.
If the energy for the reaction is supplied by an RF electric field which powers a gas plasma discharge in the deposition chamber near the substrate surface, then the substrate temperature need not be as high. In such processes (referred to as Plasma Enhanced CVD (PECVD), the substrate temperature is 300.degree. C. or less. However, in PECVD, the substrate and the film formed thereon are immersed in the plasma discharge, which will potentially damage the substrate and the film during growth. Other disadvantages of CVD processes include the reaction and nucleation of the reactants in the gas phase. When growing composite films, the reaction between the reagent gases must occur in the film to obtain a film with the desired uniformity. If the reaction occurs in the gas phase, the reaction products precipitate onto the substrate surface and contaminate the growing film.
Downstream CVD processing has been employed to avoid the problems associated with permitting the plasma to contact the substrate. As its name implies, in downstream plasma processing, the CVD reagent gas is introduced into the reaction chamber downstream of the plasma. Physical Vapor Deposition (PVD) has also been utilized. PVD includes methods of evaporation (metallizing), sputtering, molecular beam epitaxy, and vapor phase epitaxy. PVD processes typically occur in a chamber evacuated to a pressure of less than 10.sup.-6 Torr. The material from which the film is formed is present in the chamber in bulk solid form. The material is converted from the solid, condensed phase to the vapor phase using thermal energy (i.e. evaporation) or momentum transfer (i.e. sputtering). The atoms or molecules of the material condense on the substrate (and the chamber walls) as a thin film. If the pressure becomes too high, the molecules or atoms start to collide with a frequency that reduces the deposition rate.
Reactive evaporation and sputtering processes involve the intentional introduction of oxygen, nitrogen, or other reactive gas to form a thin film of an oxide, nitride or other composite material. In such processes, the pressure must be carefully controlled to maintain an effective environment for deposition. If the pressure is too high, the atoms or molecules will react in the gas phase. Furthermore, the source of the reactive atoms or molecules is subject to contamination by the reactant gases if it is allowed to come into contact with these gases.
U.S. Pat. No. 5,356,672 to Schmitt, III et al. describes a method for forming a thin film of a composite material on a substrate. The composite film is formed by the interaction between a first reagent gas and a second reagent gas. The interaction is controlled by translating a substrate from a first position where it is subjected to the discharge from a first gas jet apparatus to a second position where it is subjected to the discharge from a second gas jet apparatus. In one embodiment, the discharge from the first gas jet apparatus contains a mixture of one reactive species and a carrier gas. The discharge from the second gas jet apparatus contains a mixture of a second reactive species and a carrier gas. This method requires the substrate to be moved from the first position to the second position in a time less than the time required to complete film formation and it also requires that the wafer be moved frequently from the first position to the second position. In a second embodiment, the discharge from the gas jet apparatus contains a mixture of the first and second reactive species and the substrate is scanned or otherwise moved to subject all portions of the substrate to the discharge.
Such methods require that the substrate be subjected to background gases in the chamber as it is moved. The background gases typically contain molecular or atomic species that compete with the desired interaction between the first and the second gases. These competing interactions, if they occur to any significant extent, will have an undesired effect on the composition of the film. Furthermore, the requirement for fast and frequent translation is mechanically rigorous. Therefore, a simpler and more effective process solution is required.